A typical timber harvesting machine is a feller buncher which includes a four-wheel drive tractor and an attachment. The attachment may be bunching shear. It may also be a saw, a brush cutter, a stump grinder or other forestry or industrial attachment. An example may be seen in the Model 511EX feller buncher manufactured and sold by the forestry and industrial equipment division of Blount, Inc. in Owatonna, Minn., assignee of the present application.
The Model 511EX tractor normally is powered by a 185 horsepower diesel engine. The engine supplies motive power to each of the four wheels through a hydrostatic transmission or hydrostat. The hydrostat includes a variable displacement hydraulic pump which drives a variable displacement hydraulic motor. The hydraulic motor in this tractor is a bent axis motor manufactured and sold by the Sauer-Sundstrand Company of Ames, Iowa, but other similar motors may be used.
The bent axis motor has two settings, a minimum angle setting providing lower displacement and a maximum angle setting providing higher displacement. The output or drive shaft from the motor rotates at a speed determined by the angle setting of the motor and the volume of hydraulic fluid supplied by the pump. The direction of rotation of the motor and the forward or reverse travel of the tractor is dictated by the direction of pump output. The operator controls this from the cab through foot pedals and a hydraulic control circuit.
The bent axis motor drives the tractor's front and rear axle drive shafts through a two-speed, helical gear transfer case. Shifting gears in the transfer case between a low gear and a high gear is effected by the operator through an electrical control circuit and a solenoid operated valve which directs hydraulic fluid to and from an actuator cylinder. The cylinder dictates the position of a gear shift fork. A three-way switch in the cab permits the operator to move the three-way, two-position valve between a low gear position, a high gear position and a neutral position.
Standard operating procedure for such feller buncher machines involves stopping the tractor before shifting gears. This procedure is recommended by the manufacturer because, it has been found, shifting the machine while it is being driven frequently results in damage to the transfer case, including the gears and/or the shifting forks, and it consistently results in excess wear of these components. In addition, when the machine is shifted while it is moving, the synchronizing shaft in the hydrostat's bent axis motor is suddenly subjected to additional stress due to torque changes and this, it has been found, may result in damage to the shaft.